1. Field of the Invention
The present invention relates to an image signal processing apparatus which binary encodes an analog image signal, for example, obtained by scanning an image, and produces a digital image siganl.
2. Description of the Prior Art
It is already known how to photoelectrically read an image, perform electrical processing of an obtained image signal, and transmit or store the obtained image signal.
Electrical processing of an image of a microfilm has also been proposed. According to this electrical processing, a microfilm is irradiated with light from a light source. Light which is transmitted through the microfilm is focused on a light-receiving surface of an image sensor (e.g., a CCD image sensor or the like, to be referred to as a CCD hereinafter to perform photoelectric conversion. Thus, an image of the microfilm is obtained as an analog signal. The obtained analog signal is converted into a binary image signal.
When the analor signal obtained from the CCD is binary encoded, a method is generally adopted wherein the obtained analog signal is compared with a reference fixed potential. This reference fixed potential is called a slice level. However, with such a method, a correct digital image signal is difficult to obtain, for the following reasons. In other words, in some images, the range of gray level changes falls above or below the fixed potential, so that these changes cannot be detected with reference to such a reference fixed potential. When a CCD is used, the amount of light received by the CCD is generally obtained as a product of the amount of light output by light source used and the irradiation time. Therefore, if the light source ON time is kept constant, the amount of light received and the light output of the light source are proportional to each other. A portion of a negative film corresponding to a large character has a high transmittance and receives a large amount of light. However, a portion of such a negative film corresponding to a small character has a low transmittance, and therefore receives a small amount of light. For this reason, when an image including both a large character and a small character is read by a CCD and an analog image signal is obtained, the analog image signal has a wide dynamic range. Therefore, when the analog image signal obtained from the CCD and a fixed slice level are compared to obtain a digital signal, a tip of a large character is rounded, and a small character may vanish altogether. When such a signal is used for image reproduction, the quality of the reproduced image is low.